KVM: SVM: Ensure target pCPU is read once when signalling AVIC doorbell

Ensure vcpu->cpu is read once when signalling the AVIC doorbell.  If the
compiler rereads the field and the vCPU is migrated between the check and
writing the doorbell, KVM would signal the wrong physical CPU.

Functionally, signalling the wrong CPU in this case is not an issue as
task migration means the vCPU has exited and will pick up any pending
interrupts on the next VMRUN.  Add the READ_ONCE() purely to clean up the
code.

Opportunistically add a comment explaining the task migration behavior,
and rename cpuid=>cpu to avoid conflating the CPU number with KVM's more
common usage of CPUID.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -675,10 +675,18 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
 	smp_mb__after_atomic();
 
 	if (avic_vcpu_is_running(vcpu)) {
-		int cpuid = vcpu->cpu;
+		int cpu = READ_ONCE(vcpu->cpu);
 
-		if (cpuid != get_cpu())
-			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpuid));
+		/*
+		 * Note, the vCPU could get migrated to a different pCPU at any
+		 * point, which could result in signalling the wrong/previous
+		 * pCPU.  But if that happens the vCPU is guaranteed to do a
+		 * VMRUN (after being migrated) and thus will process pending
+		 * interrupts, i.e. a doorbell is not needed (and the spurious
+		 * one is harmless).
+		 */
+		if (cpu != get_cpu())
+			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
 		put_cpu();
 	} else
 		kvm_vcpu_wake_up(vcpu);